This invention generally relates to methods and apparatus for starting a welding arc. In particular, the invention relates to methods and apparatus for starting a TIG welding arc.
Many methods of welding are known in the art, each with its own advantages and disadvantages. Common welding processes include gas welding, oxyacetylene brazing and soldering, shielded metal arc welding (SMAW) or “STICK” welding, gas metal arc welding (GMAW) or “wire feed” welding, gas tungsten arc welding (GTAW) or “TIG” welding, and plasma cutting. TIG welding is perhaps the cleanest, most precise of all hand-held welding operations. Although the method and apparatus of the present invention is preferably directed to a TIG welding operation, one skilled in the art will appreciate that the present invention may have applications for many other welding processes.
A conventional TIG welding process will now be described with reference to FIG. 1. In TIG welding, a concentrated high-temperature arc is drawn between a non-consumable tungsten electrode 10 and a workpiece 14, workpiece 14 being connected to the output of a welding power source (not shown) via a work clamp 24. Electrode 10 is nested in a torch 16, the torch including a shielding gas source 18, such as a cup, to direct a shielding gas 20, such as argon, helium, a mixture thereof, or other inert or non-inert gases, to a welding site 22 on workpiece 14. Torch 16 receives a flow of shielding gas 20 from a gas tank (not shown). In accordance with a known technique, the welder may strike an arc by touching or scraping the electrode 10 against the workpiece 14 to close a circuit between the electrode 10 and the work clamp 24. As electrode 10 is drawn away from the workpiece 14, an arc 12 is initiated. The welder then feeds a bare welding rod 26 to welding site 22. More precisely, the tip of the welding rod 26 is dipped into the weld puddle. The arc that crosses the gap from the electrode tip to the workpiece causes underlying workpiece material at the welding site to melt, thereby creating a molten puddle 28. During a single welding pass, the arc 12 and the welding rod 26 must be moved in unison in order to effect a weld bead. The displaced arc leaves the molten puddle 28 in its wake. The portion of the molten puddle furthest from the arc hardens continuously to leave a weld bead 30 joining two pieces of metal.
Numerous problems persist with the aforementioned physical method of striking an arc because the tip of the tungsten can contaminate the weld due to touching or scraping the electrode against the workpiece. Often, due to arcing a piece of the tip remains in the molten puddle and contaminates the weld. Also, the welder must then resharpen or replace the electrode. Not only does this process inconvenience the welder, but it also wastes time and resources, which ultimately imparts a higher cost to each weld.
One known solution to the above problems has been to use a high-frequency signal to initiate and maintain the arc. A high-frequency signal ionizes the shielding gas, allowing the welding power to jump the gap between electrode and workpiece. However, high frequency, too, has its drawbacks. The high-voltage, low-amperage noise from the high-frequency circuitry often causes electrical interference with surrounding equipment, making its use unacceptable in certain applications. Also, the high-frequency signal can be tough on TIG torches and work leads because the high voltage causes a stress to be applied to the insulation of the weld cables.
Another arc starting method that avoids the problems associated with the scratch start is the “lift” arc method. Lift arc starting involves touching the electrode to the workpiece without the necessary scraping to generate a spark. Some known lift arc methods utilize a separate low-current power circuit, in addition to the power circuits already present in a welding device, to create a small monitoring voltage between the electrode and work clamp. Control circuitry monitors the voltage between the electrode and work clamp and, when a short is detected (i.e., the electrode has been touched to or brought in close proximity with the workpiece), enables the power circuit to provide an initial regulated current to warm, but not melt the electrode. When the control circuitry detects a significant torch-to-workpiece voltage (i.e., the electrode is no longer touching or is not in close proximity to the workpiece), the control circuitry enables the power circuit to provide full user-selected welding power. However, the separate power circuit required to provide the small monitoring voltage leads to additional cost and complexity of the circuitry in the welding power source. Furthermore, some lift arc start methods fail to reliably regulate the output current level before and after the short is detected. An improved “lift” arc technique, directed to overcoming the foregoing disadvantages, is disclosed in U.S. Pat. No. 6,034,350. Still some welding procedures require that the tungsten not touch the workpiece.
Another known solution, disclosed in U.S. Pat. No. 6,075,224, is to start a welding arc by applying an arc starting signal to ionize the shielding gas before enabling welding output power. The welding device disclosed in U.S. Pat. No. 6,075,224 comprises a power circuit to provide welding power, a shielding gas source to provide a shielding gas at a welding site disposed between an electrode and a workpiece, an arc starter circuit (e.g., a high-frequency start circuit) to apply an arc starting signal to ionize the gas, and a controller coupled to a control input of the power circuit. The arc starting steps are also controlled by the controller. First, the controller enables a flow control meter to begin supplying shielding gas to the welding site. When the pre-flow period has expired, the controller enables the arc starter circuit, which generates an arc starting signal that is provided to the power output for a predetermined period of time during which the resulting arc ionizes the flow of shielding gas particles. The starting arc is not suitable for welding. A predetermined time after the arc starting signal is applied, the controller enables the power circuit such that welding power is provided and an arc suitable for welding is drawn between the electrode and the workpiece.
There is an ongoing need for further improvements in methods and apparatus for initiating and maintaining a TIG or other welding arc.